Now, let me start by saying I understand there's only 11 months left until the release of Skyrim, so all the main facets of the game are created, no major deals are being added. All our 'cool ideas' for Glaciators, Cannibalism, and making it more like Dragon Age are silliness left for the main-streamers. They're not going to be in Skyrim. However, if we have some fun ideas, I think we should share them as possible ideas for DLCs and Added content.
Now, a lot of guild ideas are being tossed around, some serious, some not so serious, but I think we're over-looking a major facet of life: architecture. A lot of you say you love to roleplay and you don't want to be shoehorned into a role. You don't want to have to be the Chosen One. (I know a significant number of you want to be the chosen one. Meh) However, you are shoe-horned either way in the fact that the offered guils we know are Fighters, Mages, and Thieves. How's about some actual guilds from the middle ages? We've yet to see how crafting will work in Skyrim (other than Alchemy) so I can't suggesting a Smithing Guild or a Leahter-Tanning guild.
Instead I offer a Stone Mason's Guild.
Masonry is an art and a science, with apprentice and master masons making up its ranks. In the Middle Ages, the Masons were highly revered for their art, comissioned to build noble villas, cathedrals, and castles. In Oblivion: the Imperial Sewers, the various forts and towers, and cultural buildings, these are all the realm of a mason. As a mason, you would use alchemy to trasnmute and create various materials for building (perhaps destruction fireballs to bake the bricks). To add more skills to this Guild, we could perhaps involve conjuration in the summoning of servitors to aid in the building projects, instead of laborers.
Quests might involve meeting clients and convincing them to invest in your Guild, pushing out independent competitors, satisfying commisions by collecting supplies for those lovely fetch quests that grace every RPG. As we increase in the ranks, we could be sent out to the frontier, perhaps working for the Imperial armies (what's left of them, following the rise of Titus Mede and the fall of the Empire) to assembly towers and forts, offering strategies to quell the warring cities of the Nords. Finally, we could be rewarded with a keep or schloss of our own 'making,' like those tiny DLCs that added manors and castles and hideouts back in Oblivion's time.
Of course, this would all be in the form of a DLC for Skyrim, or some sort of add-on. If people are interested, I could go into further detail.